It is a very good question and as he points out in his blog post on the subject, it seems like a lot of overhead for very little benefit. For the most part, he is right.
To my defense, and as Greg points out, I would not start with such a test when writing email functionality. I would start with the mock email provider and follow the typical Red-Green TDD pattern of development. However there are cases where this approach does not test enough and this testing server was necessitated by some real world scenarios I ran into.
For example, in some situations, it is very important to understand the exact format of the raw SMTP message that is sent. Some systems actually use email from server to server to kick off automated tasks. In that situation, it helps to know that the SMTP message is formatted as expected by the receiving server. For example, you may want to make sure that the appropriate headers are sent and that the message is not a multi-part message. This approach lets you get at the raw SMTP message in a way that the mock provider approach cannot.
A more common issue is when sending mass mailings such as newsletters to subscribers. At one former employer, we had real difficulty getting our emails to land in our user’s mailbox despite adhering to appropriate SPAM laws and only mailing to subscribed users.
It turns out that actually landing a mass mailing even to users who want
the email is very tough when dealing with Hotmail, yahoo, and AOL
accounts. Something as seemingly innocuous as the
value can trigger the spam filters.
In this case, this very much falls under the rubrik of an integration test, as I am testing the actual mailing component in use. But I am not only testing the particular mailing component. I am also testing that my code uses the mailing component in a correct manner.
So in answer to the question, Where’s the red bar? The red bar comes
into play when I write my unit test and asser that the
is missing. The green bar comes into play when I make sure to remove the
header. I could probably test this with a mock object as well, but I
have been burnt by mailing components that did not remove the X-Mailer
header but simply set the value to blank, when I really intended it to
be removed. That is not something a mock object would have told me.